N.F.L.

The study, which began in 2007 and was to last through at least next year, was criticized for matters ranging from poor statistical sampling to conflicts of interest. For example, a co-chairman of the league’s committee on concussions, Dr. Ira Casson, was performing every neurological examination while consistently discrediting evidence linking football to cognitive decline and dementia in N.F.L. retirees.

The league spokesman Greg Aiello said in an e-mail message Friday that the study of retired players “is on hold for now as we explore having the study supported through an academic medical institution.”

Aiello said any adjustment to the study protocol would be considered by the committee’s new chairman or chairmen, who will probably be selected next month.

The last three years have featured dozens of reports of players in severe cognitive decline, and several studies have found drastically heightened rates of dementia among retirees.

In September, a phone survey commissioned by the league reported that players and their families were reporting diagnoses of Alzheimer’s and other memory-related diseases at five times the national rate for men age 50 and above, and 19 times the normal rate for men age 30 to 49.

The league, with Casson its primary voice, said that those studies and several that had preceded them were too flawed to be considered evidence of any link. They said that the Casson-led study would become the definitive work on the subject.

Outside experts objected when they learned some of that study’s details. Epidemiologists said that its primary design  to examine 120 N.F.L. players and 60 men who played only through college, spread across ages from 30 to 60  could not discover any heightened prevalence of dementia.

Players like the Hall of Fame linebacker Harry Carson joined experts in health policy in questioning how Casson could be allowed to conduct all neurological examinations personally. They and many others said a study financed by the N.F.L. and run by its committee doctors was a conflict of interest.

“The design suffers from a total lack of statistical power to detect an effect if one truly exists,” Amy Borenstein, an epidemiologist at the University of South Florida, said before the Congressional hearing.

During the hearing, Sanchez pointed to how N.F.L. players were being compared to college players.

“That is sort of like comparing two-pack-a-day smokers with one-pack-a-day smokers to see what the differences are,” she said, “instead of two-pack-a-day smokers with the general population to see whether there is an increased risk of the activity that they are participating in to their health.”

Several members of Congress criticized Casson, a neurologist in Queens and a member of the N.F.L.’s concussion committee since 1994, for not appearing before the committee and testifying about his study’s design. He has agreed to testify at a second hearing Jan. 4 in Detroit.

After the resignations of Casson and Viano, the functional chairman of the league’s concussion committee is Dr. Elliot Pellman, the Jets’ team physician and the group’s longtime chairman before he resigned in early 2007. In a telephone interview Friday, Pellman declined to comment on the status of the retired player study.

The viability of any study conducted by the N.F.L. committee, regardless of its leadership, is in doubt. Sanchez has said that the neurological exams on the approximately 60 players who have already participated should be discarded because, she said, “Dr. Casson’s mind-set wasn’t impartial from the outset.”